This invention relates to a decoding method of a telecommunication network, in which method: information frames are received and data contained in them is buffered until the amount of the buffered data corresponds to a predetermined amount, the buffered data is decoded, a parity check is made for the buffered data, and the buffered data is transmitted further in the telecommunication network.
This invention is specifically connected to the GSM mobile telephone system (Groupe Special Mobile), and more precisely to a method by which the quality of the connection between the mobile, that is, the subscriber unit, and the mobile services switching center can be improved.
In situations in which there is an active traffic channel in a base station but the mobile does not transmit radio signals to the base station on the connection continuously, the base station has no way of detecting from a received signal whether the subscriber unit has transmitted the frame or not. Therefore, the base station constantly receives noise from the radio path, detects and decodes it, and sends speech blocks further in TRAU-frames to a transcoder.
The above-mentioned situation occurs in the GSM system at least in connection with a handover when the handover is made between two base stations, or when a subscriber unit uses discontinuous transmission, that is, is in a DTX state. In the DTX state the-subscriber unit sends one speech frame of 20 ms in length in accordance with the GSM specifications, after which it has a pause of 460 ms in length before transmitting the next speech frame.
According to the GSM specifications, the base station has to be able to detect 99,8% of all defective speech blocks sent via the base station. This detection is based on a parity check made in connection with the decoding of speech blocks. Because in the above mentioned special situations all the speech blocks received from the radio path by the base station are defective, a defective speech block passes through the base station in compliance with the GSM specifications every tenth second on the average without the decoding of the base station detecting the error.